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Synthesis and characterization of pressureless sintered carbon nanotube reinforced alumina nanocomposites
Materials Science & Engineering A 578 (2013) 422–429
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Synthesis and characterization of pressureless sintered carbon nanotube reinforced alumina nanocomposites N. Bakhsh a,n, F. Ahmad Khalid a, A. Saeed Hakeem b a b
Faculty of Materials Science and Engineering, GIK Institute of Engineering Sciences and Technology, Topi, KPK, Pakistan Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran, Kingdom of Saudi Arabia
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2012 Received in revised form 10 March 2013 Accepted 6 April 2013 Available online 23 April 2013
This paper presents sintering and characterization of multiwalled carbon nanotube reinforced alumina nanocomposites and their comparison with the sintering behavior of pure alumina. Two types of composites were prepared by different techniques and both contained 1, 2 and 3 wt% of as-received and functionalized carbon nanotubes which were synthesized by pressureless sintering. The mixing and dispersion of carbon nanotubes in alumina were done by a novel technique of gas purging sonication. Varying percentages of carbon nanotubes in the composites were compacted using a uniaxial press followed by sintering at 1600 1C in flowing argon. Nanocomposites with 1 wt% carbon nanotubes gave 98.5% relative density with no degradation of carbon nanotubes. Moreover, it also resulted in an increase in fracture toughness from 8.1% and 9.4% and Young's modulus by 5% and 7% when compared to asreceived and functionalized carbon nanotube nanocomposites respectively with respect to pure alumina. This investigation has shown that the densification can be achieved without degradation of carbon nanotubes at elevated temperatures in the carbon nanotube–alumina nanocomposites sintered by the conventional route. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials Composites Sintering Mechanical characterization
1. Introduction Since their discovery in 1991 [1], carbon nanotubes (CNTs) have gained an extreme importance in the field of advanced materials due to their outstanding mechanical, electrical and thermal properties [2]. The ultra-high modulus of elasticity and high tensile strength of CNTs make them a strong reinforcement candidate for polymers, metals, alloys and ceramic nanocomposites. Although the CNTs exhibit exceptional mechanical properties, but their response toward reinforcing the ceramics is far below than the expected due to high aspect ratio and strong Van der Waals forces between them. These properties tangle the CNTs and impose serious limitations to achieve homogeneous dispersion, adequate densification and desired properties for various commercial applications [3–7]. Previous work [8–11] focused on achieving good dispersion of CNTs in the polymer, metal or ceramic matrices using different techniques such as: shear mixing, ball milling, ultra-sonication, in-situ growth and surface functionalization of CNTs. The demand for advanced composite materials comprising of high performance characteristics and improved functional properties is always rising [12]. The prospective use of CNT-reinforced ceramic nanocomposites for various engineering
n
Corresponding author. Tel.: +92 938 271858; fax: +92 938 271865. E-mail address: [email protected] (N. Bakhsh).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.04.020
applications has unlocked an interesting area of research. Alumina, along with other ceramics like zirconia, silicon nitride, tungsten carbide, titania, etc., is not only being used as a biomedical material but also for high temperature applications in aerospace and automobile industries. This is due to its high hardness, good wear resistance, thermal/electrical insulation properties and excellent chemical inertness. But the main problem associated with alumina is its low fracture toughness which sometimes hinders its application as a structural material [13,14]. Various types of particulate, whisker and fiber reinforcements have been used to improve the fracture toughness of alumina in the last few decades [15–18]. In this regard, CNTs are achieving a lot of attention as a reinforcement candidate due to their good thermal stability up to 1800 1C [19,20] and excellent mechanical properties after sintering with alumina. CNTs connect the alumina grain boundaries and retard grain growth resulting in the grain refinement during the sintering at elevated temperatures which help in the improvement of fracture toughness, Young's and shear modulii [20–22]. The toughening mechanism of CNT–alumina nanocomposites is mainly attributed to the crack deflection at CNT–alumina interface, crack bridging and CNT pull-out from alumina grains [23,24]. Zhan et al. [24] reported a three times increase in fracture toughness of 10 vol% single-wall CNT–alumina composite over the monolithic alumina using a spark plasma sintering technique. Similarly, a significant improvement in fracture toughness was observed in 4 vol% MWCNT–alumina composites after hot pressing [13].
N. Bakhsh et al. / Materials Science & Engineering A 578 (2013) 422–429
Zhang et al. [25] reported an improvement in flexural strength and fracture toughness of MWCNT–alumina composites after pressureless sintering at 1500 1C for 2 h. Generally, the sintering of CNT–alumina composites has been carried out from 1200 to 1800 1C both by conventional and nonconventional techniques. However, more attention has been paid on the sintering techniques such as: hot pressing (HP), hot isostatic pressing (HIP), microwave sintering and spark plasma sintering (SPS). Most of these processes require a simple geometry for the sintered products [18,20,26–29]. On the contrary, less attention is given to the conventional pressureless sintering for the synthesis of CNT–alumina nanocomposites which can have major production benefits [25]. This work demonstrates that CNT reinforced alumina nanocomposites can be produced by pressureless sintering to retain large concentration of CNTs in the samples. Optimization of CNTs concentration is made to illustrate that 1 wt% CNTs can produce better properties in the functionalized condition as compared to the as-received condition. A new synthesis technique is adopted and processing parameters are optimized and presented to explain the role of different amounts of CNTs in alumina nanocomposites.
2. Experimental procedure 2.1. Materials Carbon nanotubes with diameter of 10 to 40 nm and purity level of around 90% were procured from Sun Nanotech, China, while α-alumina powder having submicron particle size was received from Nabond Technologies, China. Nitric acid (HNO3), sulfuric acid (H2SO4) and hydrochloric acid (HCl) were purchased from Merck, Germany. 2.2. Functionalization of CNTs For the purification and functionalization, as-received CNTs were refluxed in a mixture of concentrated sulfuric acid (H2SO4) and nitric acid (HNO3) with a ratio of (3:1) solution at 110 1C for 2 h. The filtrate was thoroughly washed with de-ionized water (until the pH approaches to 7) to get a glossy cake of functionalized CNTs which was dried at 120 1C for 24 h in a drying oven. 2.3. Fabrication of composites The functionalized CNTs (1, 2 and 3 wt%) and alumina powder were dispersed separately through sonication in de-ionized water as a mixing aid medium. These were further ball milled at 200 rpm for 2 h in a planetary ball mill to get the homogeneous and deagglomerated suspensions. The same alumina suspension was stirred magnetically with the addition of a few drops of 35% HCl to maintain the pH at 4.4. Subsequently, the functionalized CNTs suspension was added drop wise into alumina suspension to get a homogeneous composite suspension. A new mixing technique was employed to achieve better dispersion of CNTs in the alumina matrix. After ball milling of as-received CNTs and alumina powder in 200 ml of de-ionized water, the suspension was further mixed with a unique dispersion technique of gas purging sonication (GPS), which consisted of simultaneous purging of nitrogen gas (0.3–0.5 l/min) and sonicating the solution until a thick slurry was achieved. This technique of purging and sonication (Fig. 1) for the de-agglomeration of CNTs and alumina resulted in a uniform mixture of nanocomposite powder. This process for the mixing and de-agglomeration of CNTs has not been reported previously.
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Fig. 1. Schematic diagram of gas purging sonication.
The resultant composite slurries were dried at 120 1C for 24 h. The powder was passed through a sieve (250 mesh) to minimize the particle size variations in the nanocomposite mixture and was uniaxially pressed at 375 MPa in a steel die to get green pellets. Pellets were dried overnight at 100 1C prior to sintering at 1600 1C for 15 min in flowing argon atmosphere using a high temperature graphite furnace. The heating and cooling rates were adjusted at 10 1C/min and 5 1C/min respectively. 2.4. Test method Microstructural analysis was carried out using a scanning electron microscope (SEM) Philips (XL30), and a field emission electron microscope (Tescan Lyra-3). X-ray diffraction analysis (XRD) was performed using an X-ray diffractometer (Philips PW3710) with Cu-Kα radiations. A Microtrac S3500 Particle Analyzer was used for the analysis of alumina powder. A microhardness tester with a Vickers' sensor was used to measure hardness at indentation load of 200 g with a dwell time of 15 s. A Vickers hardness tester (DVK-2) was used for fracture toughness measurements using a load of 10 kg. Theoretical densities were calculated by using rule of mixture. Archimedes' principle was used for the calculation of densities of sintered samples by immersing the specimen in de-ionized water. The elastic modulii of the samples were calculated using a standard pulse-echo technique. The shear modulus (G) was calculated using the following equation [30]: G ¼ ρðV T Þ2 where ρ is the density of the material and VT is the transverse wave velocity. The following equation was used to calculate the fracture toughness [13,31]: 1=2 E P K IC ¼ 0:016 H c3=2 where E is the elastic modulus, H is the hardness, P is the applied load while c is the radial crack length due to indentation.
3. Results and discussion 3.1. Morphology and functionalization of carbon nanotubes Fig. 2(a) shows the SEM image of as-received CNTs which have various diameters ranging from 10 to 40 nm. Impurities like amorphous carbon, and nanocatalyst transition metal particles are also present. These impurities are not desirable in the production of CNT–alumina composite, therefore, purification of
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Fig. 2. SEM image showing morphology of (a) as-received CNTs, (b) purified CNTs and (c) FTIR spectrum of functionalized CNTs.
CNTs was carried out to produce composites and Fig. 2(b) shows purified and functionalized CNTs with the removal of amorphous carbon and metal particles. Fourier transformed infrared (FTIR) spectrum of functionalized CNTs (Fig. 2(c)) shows stretching vibration of O–H and C–H bonds between 2800 and 3500 cm−1 which are related to hydroxyl and carboxylic groups. The peak at 1600 cm−1 is the characteristic peak of C–C bonding due to carbon skeleton. The acid treatment also provides a stretching vibration of C–O at approximately 1450 cm−1, which indicates the carboxylic group appearance due to surface oxidation. These functionalized groups are responsible for the improved dispersion and interfacial bonding of CNTs with the alumina matrix [32,33]. 3.2. Morphology and particle size analysis of alumina powder Fig. 3(a) shows morphology of as-received alumina powder. The alumina particles are irregular in shape with size ranging from less than 100 nm to 500 nm [34]. Fig. 3(b) shows the particle size distribution of alumina powder. The average particle size of alumina powder is approximately 0.41 mm. 3.3. Sintering and microstructural phases Fig. 4 presents the surface topography of pure sintered alumina and its composites with as-received and functionalized CNTs. Fig. 4 (a) shows that there are no pores at the surface of monolithic alumina sintered at 1600 1C. However, the porosity at the surface is relatively increased by the addition of 1 wt% as-received and functionalized CNTs in alumina which are evident by small cavities in Fig. 4(b) and (c) respectively. The same trend is observed on further addition of CNTs. The comparative sintered density of monolithic Al2O3 to its theoretical density is 99% as shown in Fig. 5. The pores removal
and mass transportation due to bulk diffusion are major reasons for high density of alumina. Sintering of CNT–-alumina at 1600 1C results in reduced density with increasing amount of CNTs in the matrix. The densities of 1, 2, and 3 wt% functionalized CNT-composites are reduced to 98.5%, 96.8% and 94% respectively, as compared to the corresponding theoretical densities showing better results compared to previous one [25]. This decrease in density can be attributed to nanosized pores in CNTs clusters and alumina powder particles which persist during compaction and sintering and consistent with the previous work [35,36]. On the other hand, decreasing trend in the density with increasing as-received CNTs content is greater as compared to functionalized CNTs in the composites as shown in Fig. 5. This reduction in density is due to the presence of as-received CNTs clusters at the grain boundaries. These agglomerates act not only as solute stagnations between grain boundary interfaces but also as the source of retardation to grain growth at the sintering temperature [25,35,37]. It is observed that up to 1 wt% as-received and functionalized CNTs, there is a marginal difference in the densities of nanocomposites. However, further increase of CNTs preserves more obvious difference in the densities of both. This is because of higher degree of agglomeration of as-received CNTs in the alumina matrix compared to functionalized CNTs. Raman spectroscopic analysis of CNTs and CNT–alumina composite was carried out to observe any damage to CNTs during the sintering process. Two bands are observed at Raman shifts of ∼1350 cm−1 and ∼1580 cm−1 which correspond to the characteristic values for D-band (defect mode) and G-band (graphite mode) respectively as shown in Fig. 6. The Raman intensities of D-band and G-band for as-received CNTs are almost equal. However, the value of ID/IG ratio increases which confirms that the CNTs are intact structurally after sintering. A shift in the G-band is not observed in comparison with the as-received CNTs, which shows
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Fig. 3. (a) SEM image showing morphology of alumina powder, and (b) particle size distribution of alumina powder.
the absence of thermal residual stresses after the sintering process [23,38,39]. Fig. 7(a) shows the XRD spectrum of pure alumina powder in which high value of full width at half maximum (FWHM) of different planes is evident due to submicron particle size. After sintering alumina powder at 1600 1C, the crystallite size is increased which is obvious from reduced values of FWHM of different planes as shown in Fig. 7(b). The CNTs addition does not produce any new phase in the alumina nanocomposites after sintering at 1600 1C [13] as shown in Fig. 7(c) and (d). 3.4. Mechanical properties Fig. 8(a) shows the comparison of hardness values of alumina matrix nanocomposites containing as-received and functionalized CNTs. Hardness of monolithic alumina is 25 GPa after sintering at
Fig. 4. SEM images showing surface topography of (a) monolithic alumina, (b) Al2O3–1 wt% as-received CNTs and (c) Al2O3–1 wt% functionalized CNTs.
1600 1C. By adding 1 wt% functionalized CNTs and as-received CNTs in alumina matrix, hardness values decrease by 11% and 15% respectively. On further increasing the concentration to 2 wt% CNTs, the hardness values follow the same decreasing trend with 30% and 46% for both kinds of CNTs as compared with monolithic alumina, however, the rate of decline in hardness is less for 3 wt% functionalized CNTs as compared to as-received CNTs. This may be attributed to agglomeration at higher concentrations
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Fig. 5. Comparison of sintered density of Al2O3/as-received CNT and Al2O3/ functionalized CNT nanocomposites.
Fig. 6. Raman spectra of as-received CNTs and CNT–alumina nanocomposite.
of CNTs, reduced cohesion of grain boundaries due to presence of CNTs intergranular phase and the increased porosity in the samples [23]. Fracture toughness has been calculated using an economic and effective technique of direct crack measurement [40]. Fig. 8 (b) shows the comparison of fracture toughness of CNT–alumina nanocomposites with monolithic alumina. Fracture toughness of pure alumina after sintering is measured to be 3.71 MPa m1/2. The addition of 1 wt% functionalized CNTs and as-received CNTs in the samples displays an increase in toughness values by 9% and 8% respectively, which is due to crack deflection, crack bridging and pull out mechanism from well dispersed CNTs in the finer alumina grains [13,25,41]. The improvement in fracture toughness is also attributed to minimization of CNTs damage during sintering and the netlike structure of CNTs in the composite [2]. A decreasing trend in the values of fracture toughness has been observed on further addition of CNTs in alumina. The reduction in fracture toughness is due to CNT-bundles inhibiting the diffusion mechanism at the grain boundaries resulting in reduced density and weak interfacial adhesion of CNTs agglomerates with the matrix [13,42]. Fig. 8(c) shows the comparison of Young's modulus of functionalized CNT and as-received CNT–alumina nanocomposites with monolithic alumina sintered at 1600 1C. Young's modulus for 1 wt% functionalized CNT and as-received CNT–alumina composites are 408 and 401 GPa respectively. These values are greater than pure alumina by 7% and 5% respectively for both kinds of CNT-composites. This increase is due to the high elastic modulus (41 TPa) and extraordinary flexibility of CNTs [25]. At 3 wt% CNTs, the modulus values start decreasing and reach to approximately half the value of pure alumina. This decreasing trend with the increase in CNTs concentrations may be due to those CNTs present in the form of agglomerates, which are unable to fully stretch during the load application and hence lowers the modulus of the composites as reported earlier [13]. The shear modulus for monolithic alumina is obtained as 163.2 GPa, which increased by 10.66% and 6% by the addition of 1 wt% functionalized CNTs and as-received CNTs respectively as shown in Fig. 8(d). The increase in shear modulus values is due to high modulus of CNTs along with good dispersion and interfacial bonding of CNTs with the matrix [5]. On increasing CNTs concentration up to 2 wt%, the shear modulus decreases by 35% and
Fig. 7. XRD spectra: (a) alumina (un-sintered), (b) alumina sintered, (c) Al2O3–1 wt% as-received CNTs nanocomposite and (d) Al2O3–1 wt% functionalized CNTs nanocomposite.
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Fig. 8. Comparison of mechanical properties of Al2O3–CNTs nanocomposites: (a) hardness, (b) fracture toughness, (c) Young's modulus and (d) shear modulus.
44% for functionalized CNTs and as-received CNTs respectively. Further increase in CNTs concentration decreases the modulus values. The ratios between shear and Young's modulii for CNT– alumina composites are in agreement with the literature [43]. It can be concluded that mechanical properties of the CNT– alumina composites initially increase with the addition of 1 wt% CNTs and then decrease at higher CNTs concentration due to agglomeration of CNTs and reduced densification which are evident in Fig. 9(e)–(h).
growth which result in grain refinement in the nanocomposites with a unique entangled network structure as described in ref. [2,20]. Similarly, the pull out pattern of CNTs (Fig. 9(c) and (d)) indicates that CNTs are embedded and tied up with the alumina grains which result in improved mechanical properties. Fig. 9(e)– (h) shows the presence of large agglomerates at higher concentration of CNTs in the composites which are responsible for decrease in the density and other mechnical properites of the nanocomposites [41].
3.5. Fracture behavior Fig. 9(a) and (b) shows fractured surface of monolithic alumina after sintering at 1600 1C. It can be seen that finer alumina particles are entrapped in the cavities between the large particles, thus filling the gap which results in the higher densification.The average grain size of monolithic alumina is 47 1 mm. The surface of pure alumina reveals largely the intergranular and transgranular fracture morphologies. SEM images of fractured samples with 1 wt% CNT–alumina nanocomposites show uniform dispersion of CNTs in the matrix (Fig. 9(c) and (d)). SEM micrographs also reveal no degradation of CNTs. However, the addition of 1 wt% CNTs in the alumina matrix also results in the decrease in the average grain size of the composites to 725 nm and 756 nm for functionalized CNTs and as-received CNTs respectively compared to pure alumina, which are much smaller as reported earlier [25]. This also confirms that the addition of both types of CNTs is creating a pinning effect at the grain boundaries and retarding the grain
4. Conclusions CNT–alumina nanocomposites were prepared using a novel mixing technique of gas purging sonication (GPS) followed by pressureless sintering. Mechanical properties such as hardness, modulus and fracture toughness showed a considerable improvement with the addition of 1 wt% functionalized CNT–alumina nanocomposites. This can be attributed to uniform dispersion of CNTs and better bond with alumina in the functionalized conditions as compared to as-received conditions. With the further addition of 2 and 3 wt% CNTs in the as-received and functionalized conditions, a decrease in the properties of nanocomposites was observed owing to agglomeration of CNTs and increase in porosity. Nevertheless stable CNTs were possible to achieve in the samples employing high temperature for shorter time using pressureless sintering.
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Fig. 9. Fractured surface morphology of (a) monolithic Al2O3, (b) monolithic Al2O3, (c) Al2O3/1 wt% as — received CNTs, (d) Al2O3/1 wt% as — functionalized CNTs, (e) Al2O3/ 2 wt% as — received CNTs, (f) Al2O3/2 wt% as — functionalized CNTs, (g) Al2O3/3 wt% as — received CNTs and (h) Al2O3/3 wt% functionalized CNTs.
Acknowledgment Higher Education Commission (HEC) of Pakistan is acknowledged for the financial support and scholarship to Nabi Bakhsh.
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Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Synthesis and characterization of pressureless sintered carbon nanotube reinforced alumina nanocomposites N. Bakhsh a,n, F. Ahmad Khalid a, A. Saeed Hakeem b a b
Faculty of Materials Science and Engineering, GIK Institute of Engineering Sciences and Technology, Topi, KPK, Pakistan Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran, Kingdom of Saudi Arabia
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2012 Received in revised form 10 March 2013 Accepted 6 April 2013 Available online 23 April 2013
This paper presents sintering and characterization of multiwalled carbon nanotube reinforced alumina nanocomposites and their comparison with the sintering behavior of pure alumina. Two types of composites were prepared by different techniques and both contained 1, 2 and 3 wt% of as-received and functionalized carbon nanotubes which were synthesized by pressureless sintering. The mixing and dispersion of carbon nanotubes in alumina were done by a novel technique of gas purging sonication. Varying percentages of carbon nanotubes in the composites were compacted using a uniaxial press followed by sintering at 1600 1C in flowing argon. Nanocomposites with 1 wt% carbon nanotubes gave 98.5% relative density with no degradation of carbon nanotubes. Moreover, it also resulted in an increase in fracture toughness from 8.1% and 9.4% and Young's modulus by 5% and 7% when compared to asreceived and functionalized carbon nanotube nanocomposites respectively with respect to pure alumina. This investigation has shown that the densification can be achieved without degradation of carbon nanotubes at elevated temperatures in the carbon nanotube–alumina nanocomposites sintered by the conventional route. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials Composites Sintering Mechanical characterization
1. Introduction Since their discovery in 1991 [1], carbon nanotubes (CNTs) have gained an extreme importance in the field of advanced materials due to their outstanding mechanical, electrical and thermal properties [2]. The ultra-high modulus of elasticity and high tensile strength of CNTs make them a strong reinforcement candidate for polymers, metals, alloys and ceramic nanocomposites. Although the CNTs exhibit exceptional mechanical properties, but their response toward reinforcing the ceramics is far below than the expected due to high aspect ratio and strong Van der Waals forces between them. These properties tangle the CNTs and impose serious limitations to achieve homogeneous dispersion, adequate densification and desired properties for various commercial applications [3–7]. Previous work [8–11] focused on achieving good dispersion of CNTs in the polymer, metal or ceramic matrices using different techniques such as: shear mixing, ball milling, ultra-sonication, in-situ growth and surface functionalization of CNTs. The demand for advanced composite materials comprising of high performance characteristics and improved functional properties is always rising [12]. The prospective use of CNT-reinforced ceramic nanocomposites for various engineering
n
Corresponding author. Tel.: +92 938 271858; fax: +92 938 271865. E-mail address: [email protected] (N. Bakhsh).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.04.020
applications has unlocked an interesting area of research. Alumina, along with other ceramics like zirconia, silicon nitride, tungsten carbide, titania, etc., is not only being used as a biomedical material but also for high temperature applications in aerospace and automobile industries. This is due to its high hardness, good wear resistance, thermal/electrical insulation properties and excellent chemical inertness. But the main problem associated with alumina is its low fracture toughness which sometimes hinders its application as a structural material [13,14]. Various types of particulate, whisker and fiber reinforcements have been used to improve the fracture toughness of alumina in the last few decades [15–18]. In this regard, CNTs are achieving a lot of attention as a reinforcement candidate due to their good thermal stability up to 1800 1C [19,20] and excellent mechanical properties after sintering with alumina. CNTs connect the alumina grain boundaries and retard grain growth resulting in the grain refinement during the sintering at elevated temperatures which help in the improvement of fracture toughness, Young's and shear modulii [20–22]. The toughening mechanism of CNT–alumina nanocomposites is mainly attributed to the crack deflection at CNT–alumina interface, crack bridging and CNT pull-out from alumina grains [23,24]. Zhan et al. [24] reported a three times increase in fracture toughness of 10 vol% single-wall CNT–alumina composite over the monolithic alumina using a spark plasma sintering technique. Similarly, a significant improvement in fracture toughness was observed in 4 vol% MWCNT–alumina composites after hot pressing [13].
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Zhang et al. [25] reported an improvement in flexural strength and fracture toughness of MWCNT–alumina composites after pressureless sintering at 1500 1C for 2 h. Generally, the sintering of CNT–alumina composites has been carried out from 1200 to 1800 1C both by conventional and nonconventional techniques. However, more attention has been paid on the sintering techniques such as: hot pressing (HP), hot isostatic pressing (HIP), microwave sintering and spark plasma sintering (SPS). Most of these processes require a simple geometry for the sintered products [18,20,26–29]. On the contrary, less attention is given to the conventional pressureless sintering for the synthesis of CNT–alumina nanocomposites which can have major production benefits [25]. This work demonstrates that CNT reinforced alumina nanocomposites can be produced by pressureless sintering to retain large concentration of CNTs in the samples. Optimization of CNTs concentration is made to illustrate that 1 wt% CNTs can produce better properties in the functionalized condition as compared to the as-received condition. A new synthesis technique is adopted and processing parameters are optimized and presented to explain the role of different amounts of CNTs in alumina nanocomposites.
2. Experimental procedure 2.1. Materials Carbon nanotubes with diameter of 10 to 40 nm and purity level of around 90% were procured from Sun Nanotech, China, while α-alumina powder having submicron particle size was received from Nabond Technologies, China. Nitric acid (HNO3), sulfuric acid (H2SO4) and hydrochloric acid (HCl) were purchased from Merck, Germany. 2.2. Functionalization of CNTs For the purification and functionalization, as-received CNTs were refluxed in a mixture of concentrated sulfuric acid (H2SO4) and nitric acid (HNO3) with a ratio of (3:1) solution at 110 1C for 2 h. The filtrate was thoroughly washed with de-ionized water (until the pH approaches to 7) to get a glossy cake of functionalized CNTs which was dried at 120 1C for 24 h in a drying oven. 2.3. Fabrication of composites The functionalized CNTs (1, 2 and 3 wt%) and alumina powder were dispersed separately through sonication in de-ionized water as a mixing aid medium. These were further ball milled at 200 rpm for 2 h in a planetary ball mill to get the homogeneous and deagglomerated suspensions. The same alumina suspension was stirred magnetically with the addition of a few drops of 35% HCl to maintain the pH at 4.4. Subsequently, the functionalized CNTs suspension was added drop wise into alumina suspension to get a homogeneous composite suspension. A new mixing technique was employed to achieve better dispersion of CNTs in the alumina matrix. After ball milling of as-received CNTs and alumina powder in 200 ml of de-ionized water, the suspension was further mixed with a unique dispersion technique of gas purging sonication (GPS), which consisted of simultaneous purging of nitrogen gas (0.3–0.5 l/min) and sonicating the solution until a thick slurry was achieved. This technique of purging and sonication (Fig. 1) for the de-agglomeration of CNTs and alumina resulted in a uniform mixture of nanocomposite powder. This process for the mixing and de-agglomeration of CNTs has not been reported previously.
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Fig. 1. Schematic diagram of gas purging sonication.
The resultant composite slurries were dried at 120 1C for 24 h. The powder was passed through a sieve (250 mesh) to minimize the particle size variations in the nanocomposite mixture and was uniaxially pressed at 375 MPa in a steel die to get green pellets. Pellets were dried overnight at 100 1C prior to sintering at 1600 1C for 15 min in flowing argon atmosphere using a high temperature graphite furnace. The heating and cooling rates were adjusted at 10 1C/min and 5 1C/min respectively. 2.4. Test method Microstructural analysis was carried out using a scanning electron microscope (SEM) Philips (XL30), and a field emission electron microscope (Tescan Lyra-3). X-ray diffraction analysis (XRD) was performed using an X-ray diffractometer (Philips PW3710) with Cu-Kα radiations. A Microtrac S3500 Particle Analyzer was used for the analysis of alumina powder. A microhardness tester with a Vickers' sensor was used to measure hardness at indentation load of 200 g with a dwell time of 15 s. A Vickers hardness tester (DVK-2) was used for fracture toughness measurements using a load of 10 kg. Theoretical densities were calculated by using rule of mixture. Archimedes' principle was used for the calculation of densities of sintered samples by immersing the specimen in de-ionized water. The elastic modulii of the samples were calculated using a standard pulse-echo technique. The shear modulus (G) was calculated using the following equation [30]: G ¼ ρðV T Þ2 where ρ is the density of the material and VT is the transverse wave velocity. The following equation was used to calculate the fracture toughness [13,31]: 1=2 E P K IC ¼ 0:016 H c3=2 where E is the elastic modulus, H is the hardness, P is the applied load while c is the radial crack length due to indentation.
3. Results and discussion 3.1. Morphology and functionalization of carbon nanotubes Fig. 2(a) shows the SEM image of as-received CNTs which have various diameters ranging from 10 to 40 nm. Impurities like amorphous carbon, and nanocatalyst transition metal particles are also present. These impurities are not desirable in the production of CNT–alumina composite, therefore, purification of
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Fig. 2. SEM image showing morphology of (a) as-received CNTs, (b) purified CNTs and (c) FTIR spectrum of functionalized CNTs.
CNTs was carried out to produce composites and Fig. 2(b) shows purified and functionalized CNTs with the removal of amorphous carbon and metal particles. Fourier transformed infrared (FTIR) spectrum of functionalized CNTs (Fig. 2(c)) shows stretching vibration of O–H and C–H bonds between 2800 and 3500 cm−1 which are related to hydroxyl and carboxylic groups. The peak at 1600 cm−1 is the characteristic peak of C–C bonding due to carbon skeleton. The acid treatment also provides a stretching vibration of C–O at approximately 1450 cm−1, which indicates the carboxylic group appearance due to surface oxidation. These functionalized groups are responsible for the improved dispersion and interfacial bonding of CNTs with the alumina matrix [32,33]. 3.2. Morphology and particle size analysis of alumina powder Fig. 3(a) shows morphology of as-received alumina powder. The alumina particles are irregular in shape with size ranging from less than 100 nm to 500 nm [34]. Fig. 3(b) shows the particle size distribution of alumina powder. The average particle size of alumina powder is approximately 0.41 mm. 3.3. Sintering and microstructural phases Fig. 4 presents the surface topography of pure sintered alumina and its composites with as-received and functionalized CNTs. Fig. 4 (a) shows that there are no pores at the surface of monolithic alumina sintered at 1600 1C. However, the porosity at the surface is relatively increased by the addition of 1 wt% as-received and functionalized CNTs in alumina which are evident by small cavities in Fig. 4(b) and (c) respectively. The same trend is observed on further addition of CNTs. The comparative sintered density of monolithic Al2O3 to its theoretical density is 99% as shown in Fig. 5. The pores removal
and mass transportation due to bulk diffusion are major reasons for high density of alumina. Sintering of CNT–-alumina at 1600 1C results in reduced density with increasing amount of CNTs in the matrix. The densities of 1, 2, and 3 wt% functionalized CNT-composites are reduced to 98.5%, 96.8% and 94% respectively, as compared to the corresponding theoretical densities showing better results compared to previous one [25]. This decrease in density can be attributed to nanosized pores in CNTs clusters and alumina powder particles which persist during compaction and sintering and consistent with the previous work [35,36]. On the other hand, decreasing trend in the density with increasing as-received CNTs content is greater as compared to functionalized CNTs in the composites as shown in Fig. 5. This reduction in density is due to the presence of as-received CNTs clusters at the grain boundaries. These agglomerates act not only as solute stagnations between grain boundary interfaces but also as the source of retardation to grain growth at the sintering temperature [25,35,37]. It is observed that up to 1 wt% as-received and functionalized CNTs, there is a marginal difference in the densities of nanocomposites. However, further increase of CNTs preserves more obvious difference in the densities of both. This is because of higher degree of agglomeration of as-received CNTs in the alumina matrix compared to functionalized CNTs. Raman spectroscopic analysis of CNTs and CNT–alumina composite was carried out to observe any damage to CNTs during the sintering process. Two bands are observed at Raman shifts of ∼1350 cm−1 and ∼1580 cm−1 which correspond to the characteristic values for D-band (defect mode) and G-band (graphite mode) respectively as shown in Fig. 6. The Raman intensities of D-band and G-band for as-received CNTs are almost equal. However, the value of ID/IG ratio increases which confirms that the CNTs are intact structurally after sintering. A shift in the G-band is not observed in comparison with the as-received CNTs, which shows
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Fig. 3. (a) SEM image showing morphology of alumina powder, and (b) particle size distribution of alumina powder.
the absence of thermal residual stresses after the sintering process [23,38,39]. Fig. 7(a) shows the XRD spectrum of pure alumina powder in which high value of full width at half maximum (FWHM) of different planes is evident due to submicron particle size. After sintering alumina powder at 1600 1C, the crystallite size is increased which is obvious from reduced values of FWHM of different planes as shown in Fig. 7(b). The CNTs addition does not produce any new phase in the alumina nanocomposites after sintering at 1600 1C [13] as shown in Fig. 7(c) and (d). 3.4. Mechanical properties Fig. 8(a) shows the comparison of hardness values of alumina matrix nanocomposites containing as-received and functionalized CNTs. Hardness of monolithic alumina is 25 GPa after sintering at
Fig. 4. SEM images showing surface topography of (a) monolithic alumina, (b) Al2O3–1 wt% as-received CNTs and (c) Al2O3–1 wt% functionalized CNTs.
1600 1C. By adding 1 wt% functionalized CNTs and as-received CNTs in alumina matrix, hardness values decrease by 11% and 15% respectively. On further increasing the concentration to 2 wt% CNTs, the hardness values follow the same decreasing trend with 30% and 46% for both kinds of CNTs as compared with monolithic alumina, however, the rate of decline in hardness is less for 3 wt% functionalized CNTs as compared to as-received CNTs. This may be attributed to agglomeration at higher concentrations
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Fig. 5. Comparison of sintered density of Al2O3/as-received CNT and Al2O3/ functionalized CNT nanocomposites.
Fig. 6. Raman spectra of as-received CNTs and CNT–alumina nanocomposite.
of CNTs, reduced cohesion of grain boundaries due to presence of CNTs intergranular phase and the increased porosity in the samples [23]. Fracture toughness has been calculated using an economic and effective technique of direct crack measurement [40]. Fig. 8 (b) shows the comparison of fracture toughness of CNT–alumina nanocomposites with monolithic alumina. Fracture toughness of pure alumina after sintering is measured to be 3.71 MPa m1/2. The addition of 1 wt% functionalized CNTs and as-received CNTs in the samples displays an increase in toughness values by 9% and 8% respectively, which is due to crack deflection, crack bridging and pull out mechanism from well dispersed CNTs in the finer alumina grains [13,25,41]. The improvement in fracture toughness is also attributed to minimization of CNTs damage during sintering and the netlike structure of CNTs in the composite [2]. A decreasing trend in the values of fracture toughness has been observed on further addition of CNTs in alumina. The reduction in fracture toughness is due to CNT-bundles inhibiting the diffusion mechanism at the grain boundaries resulting in reduced density and weak interfacial adhesion of CNTs agglomerates with the matrix [13,42]. Fig. 8(c) shows the comparison of Young's modulus of functionalized CNT and as-received CNT–alumina nanocomposites with monolithic alumina sintered at 1600 1C. Young's modulus for 1 wt% functionalized CNT and as-received CNT–alumina composites are 408 and 401 GPa respectively. These values are greater than pure alumina by 7% and 5% respectively for both kinds of CNT-composites. This increase is due to the high elastic modulus (41 TPa) and extraordinary flexibility of CNTs [25]. At 3 wt% CNTs, the modulus values start decreasing and reach to approximately half the value of pure alumina. This decreasing trend with the increase in CNTs concentrations may be due to those CNTs present in the form of agglomerates, which are unable to fully stretch during the load application and hence lowers the modulus of the composites as reported earlier [13]. The shear modulus for monolithic alumina is obtained as 163.2 GPa, which increased by 10.66% and 6% by the addition of 1 wt% functionalized CNTs and as-received CNTs respectively as shown in Fig. 8(d). The increase in shear modulus values is due to high modulus of CNTs along with good dispersion and interfacial bonding of CNTs with the matrix [5]. On increasing CNTs concentration up to 2 wt%, the shear modulus decreases by 35% and
Fig. 7. XRD spectra: (a) alumina (un-sintered), (b) alumina sintered, (c) Al2O3–1 wt% as-received CNTs nanocomposite and (d) Al2O3–1 wt% functionalized CNTs nanocomposite.
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Fig. 8. Comparison of mechanical properties of Al2O3–CNTs nanocomposites: (a) hardness, (b) fracture toughness, (c) Young's modulus and (d) shear modulus.
44% for functionalized CNTs and as-received CNTs respectively. Further increase in CNTs concentration decreases the modulus values. The ratios between shear and Young's modulii for CNT– alumina composites are in agreement with the literature [43]. It can be concluded that mechanical properties of the CNT– alumina composites initially increase with the addition of 1 wt% CNTs and then decrease at higher CNTs concentration due to agglomeration of CNTs and reduced densification which are evident in Fig. 9(e)–(h).
growth which result in grain refinement in the nanocomposites with a unique entangled network structure as described in ref. [2,20]. Similarly, the pull out pattern of CNTs (Fig. 9(c) and (d)) indicates that CNTs are embedded and tied up with the alumina grains which result in improved mechanical properties. Fig. 9(e)– (h) shows the presence of large agglomerates at higher concentration of CNTs in the composites which are responsible for decrease in the density and other mechnical properites of the nanocomposites [41].
3.5. Fracture behavior Fig. 9(a) and (b) shows fractured surface of monolithic alumina after sintering at 1600 1C. It can be seen that finer alumina particles are entrapped in the cavities between the large particles, thus filling the gap which results in the higher densification.The average grain size of monolithic alumina is 47 1 mm. The surface of pure alumina reveals largely the intergranular and transgranular fracture morphologies. SEM images of fractured samples with 1 wt% CNT–alumina nanocomposites show uniform dispersion of CNTs in the matrix (Fig. 9(c) and (d)). SEM micrographs also reveal no degradation of CNTs. However, the addition of 1 wt% CNTs in the alumina matrix also results in the decrease in the average grain size of the composites to 725 nm and 756 nm for functionalized CNTs and as-received CNTs respectively compared to pure alumina, which are much smaller as reported earlier [25]. This also confirms that the addition of both types of CNTs is creating a pinning effect at the grain boundaries and retarding the grain
4. Conclusions CNT–alumina nanocomposites were prepared using a novel mixing technique of gas purging sonication (GPS) followed by pressureless sintering. Mechanical properties such as hardness, modulus and fracture toughness showed a considerable improvement with the addition of 1 wt% functionalized CNT–alumina nanocomposites. This can be attributed to uniform dispersion of CNTs and better bond with alumina in the functionalized conditions as compared to as-received conditions. With the further addition of 2 and 3 wt% CNTs in the as-received and functionalized conditions, a decrease in the properties of nanocomposites was observed owing to agglomeration of CNTs and increase in porosity. Nevertheless stable CNTs were possible to achieve in the samples employing high temperature for shorter time using pressureless sintering.
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Fig. 9. Fractured surface morphology of (a) monolithic Al2O3, (b) monolithic Al2O3, (c) Al2O3/1 wt% as — received CNTs, (d) Al2O3/1 wt% as — functionalized CNTs, (e) Al2O3/ 2 wt% as — received CNTs, (f) Al2O3/2 wt% as — functionalized CNTs, (g) Al2O3/3 wt% as — received CNTs and (h) Al2O3/3 wt% functionalized CNTs.
Acknowledgment Higher Education Commission (HEC) of Pakistan is acknowledged for the financial support and scholarship to Nabi Bakhsh.
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